Damage to mammalian heart tissue frequently results in the loss of large numbers of cardiac cells, including mature cardiac cells, pacemaker cells, smooth muscle, and endothelial cells. Although there is some indication that cardiac cells can be regenerated in humans (Bergmann et al., 2009), the mechanism is not well understood and the process does not appear to proceed rapidly enough to repair common types of cardiac damage such as ischemia, infarction, trauma, or injury due to toxins or viral infections. Therefore, a central goal of experimental cardiac medicine has been the development of a means for regenerating cardiac cells which have been lost due to cardiac damage. Studies of the mechanisms behind the embryonic cardiogenesis have been conducted, with the aim of replicating cardiogenesis in vitro or in vivo for the purposes of regenerating damaged tissue.
Recent research has identified multipotent (Isl1+) cardiovascular progenitor (MICP) cells, which are capable of differentiating to form mature cardiac tissue. MICP cells derived from embryonic stem (ES) cells which can give rise to endothelial, cardiac, and smooth muscle cells, have been isolated (Moretti et al., 2006). Genetic studies have shown that these MICP cells express Isl1, Nkx2.5 and Flk1.
Model systems for investigating cardiogenesis include the ascidian Ciona intestinalis (Beh et al., 2007). Lineage studies have shown that the adult Ciona heart is derived from two founder cells that express Ci-Mesp, a basic helix-loop-helix (bHLH) transcription factor, and also Ci-Ets1/2 (Imai et al., 2004; Satou et al., 2004). In addition, ascidian orthologs of the conserved heart specification genes NK4 (tinman Nkx2.5), GATAa (pannier/GATA4/5/6), Hand and Hand-like (Imai et al., 2003; Davidson, 2007; Davidson and Levine, 2003; Satou et al., 2004) are expressed. Ci-Mesp-knockdown embryos did not develop heart primordia, and target inhibition of Ets1/2 activity also blocked heart specification and the expansion of the heart field. Similarly, murine homologues of Ci-Mesp, Mesp1 and Mesp2 are expressed in the early mesoderm fated to become cranio-cardiac mesoderm (Saga et al., 2000). Only the Mesp1/Mesp2 double-knockout mouse lacked any cardiac mesoderm (Saga et al., 1999; Kitajima et al., 2000), indicating a role for these genes in directing the appearance of cardiac progenitors in higher vertebrates. Redundancies of Mesp genes have made further study in embryos a daunting task.
What is needed in the art is a method of inducing cardiogenesis for the purpose of regenerating cardiac cells for the use in the treatment of damaged cardiac tissue. Reprogramming of human somatic cells into pluripotent cells by a limited number of transcriptional factors important for maintaining self renewal and pluripotency has been reported by Yamanaka's, Thomson's and Daley's groups (Takahashi et al., 2007; Yu et al., 2007; Park et al., 2008). One aspect of the present invention provides a means of reprogramming the somatic cells and directed differentiation into cardiac progenitor cells. Therefore, one embodiment of this application provides a way to test a unique regulatory paradigm that ETS2 and Mesp1 are transformative, and unlike NKX2.5 and ISL1, convert non-embryonic normal human dermal fibroblasts (NHDFs) into primary cardiac progenitors. Another aspect of the present application was to elucidate the role of Mesp1 in the regulatory hierarchy directing the appearance of cardiac progenitors.